EP0130274B1 - Process for the separation of deuterium and tritium from water by the use of ammonia or a mixture of hydrogen and nitrogen - Google Patents

Description

The invention relates to a process for the separation of at least one heavy isotope of a hydrogen-containing compound or a hydrogen-containing mixture using an ammonia synthesis and a hydrogen-nitrogen mixture.

Such a method is described, for example, in the book «NUCLEAR CHEMICAL ENGINEE-RING / Second Edition by M. Benedict, Th.H. Pigford and H.W. Levi / McGraw-Hill Book Company, pages 763 to 765.

This process is a monothermal ammonia-hydrogen exchange process, whereby almost 100% deuterium-enriched ammonia is to be obtained as the starting product for the extraction of heavy water. The exchange process in this case is associated with a process for the production of ammonia from hydrogen with a natural deuterium concentration and synthesis gas consisting of nitrogen as a quasi-parasitic process.

In contrast, the invention has set itself the goal of at least largely separating both deuterium and tritium from water containing deuterium and tritium in order to obtain water depleted in deuterium and tritium.

The water source can be, for example, river water, water from a nuclear fuel processing plant or cooling water or a moderator of a nuclear reactor plant.

The focus of the invention should be the extraction of deuterium and tritium at least largely freed water. Tritium is known to be a dangerous radioactive substance, and when using deuterium-poor water in agriculture it has been shown that such water has a growth-promoting effect.

As by-products in isotope exchange processes, in addition to the products depleted in isotopes, these by-products are enriched in these isotopes.

In the present case, the invention should make it possible to obtain water depleted in deuterium and tritium as the main product and by-products enriched in deuterium and tritium in an economical manner.

A by-product enriched with deuterium, e.g. B. hydrogen or water can then, for. B. form the feed of a conventional process for the extraction of heavy water after tritium has been separated from the product in a known manner. This tritium can, for example, be stored or, after concentration, can be used in a known manner for nuclear fusion processes.

Since an ammonia synthesis is used in the invention, the invention has also set itself the goal of a method in which ammonia is not removed from the process as a product, but only the leakage losses occurring during the method have to be covered.

The object on which the invention is based is achieved with the aid of the method steps specified in the characterizing part of claim 1. The reason for the stated mold throughputs of ammonia and water is explained at the end of the description using a numerical example.

An embodiment of the invention can consist in that the portion of the ammonia enriched in deuterium and tritium is evaporated and brought into isotope exchange with a second water stream, the water being enriched in tritium and deuterium and obtained as a by-product, and that in deuterium and tritium depleted ammonia vapor is liquefied and combined with the ammonia from the first process step before it is broken down into hydrogen and nitrogen.

A further advantageous embodiment of the invention can consist in that at least a partial amount of the liquid ammonia enriched in deuterium and tritium is brought into an isotope exchange with a second gas mixture of hydrogen and nitrogen enriched in deuterium and tritium, and further enriched in deuterium and tritium, the second gas mixture resulting from the cleavage of the enriched ammonia, a subset of this gas mixture being obtained as a by-product enriched in deuterium and tritium, and in that the subset of the gas mixture depleted in deuterium and tritium during the isotope exchange with the gas mixture, which after the first process step liquefied and split into hydrogen and nitrogen, is brought together.

In a further development of the invention, this should also make it possible to obtain the isotope ^1s N as a by-product in a more economical way than before.

The ¹⁵ N isotope could then be used as a cooling gas for gas-cooled nuclear reactors or as a so-called cover gas for light and heavy water reactors.

So far, only carbon dioxide and helium have been used as cooling gases and only helium as cover gas for light and heavy water reactors. Because of its chemical instability under radiation, carbon dioxide is only suitable in a temperature range of approx. 600 to 700 ° C. Because of its solubility in water, carbon dioxide cannot be used as cover gas for water reactors because it forms a corrosive acidic solution.

Helium is an expensive gas and requires a high degree of tightness of a plant due to its high permeability.

So far, the isotope ^1s N has not been used industrially, since it could only be obtained at an uneconomically high cost, e.g. B. by distillation of nitrogen or liquid ammonia.

The invention is explained below using two exemplary embodiments.

1 shows a schematic representation of a flow diagram for a process in which largely depleted water is obtained as the main product of deuterium and tritium and water enriched as a by-product of deuterium and tritium.

Fig. 2 shows a schematic representation of a flow diagram for a method in which water largely depleted in deuterium and tritium is obtained as the main product and hydrogen enriched in deuterium and tritium as a by-product. Nitrogen enriched in ^1s N is produced as a further by-product.

A process for obtaining deuterium and tritium-depleted water as the main product and deuterium and tritium-enriched by-product from deuterium- and tritium-containing water is carried out according to FIG. 1 in the following manner:

The feed, e.g. B. river water or deuterium- and tritium-containing wastewater is conveyed through a line 1 by means of a pump 2 into an isotope exchange tower 3 and brought into isotope exchange in countercurrent to ammonia vapor depleted in deuterium and tritium.

The water is largely depleted of deuterium and tritium. This water, which is used for agricultural or industrial purposes, still contains dissolved ammonia. This is not desirable for reasons of environmental protection, ammonia consumption, etc. The ammonia portion is therefore removed in a column 4 adjoining the lower part of the exchange tower 3, the steam required for the separation being generated with the aid of a heat source in the bottom of the column 44. This heat source can be designed, for example, as a coil 5 heated with steam. The product freed from ammonia is removed through a line 6.

The ammonia vapor, which leaves the top part of the exchange tower 3, and whose concentration of deuterium and tritium is slightly lower than the concentration of the feed water, contains water vapor, which must not be present in the further process stages.

The water vapor is therefore separated off in a rectification column 7 arranged above the exchange tower 3. The anhydrous ammonia vapor is liquefied in a condenser 8 by means of a water-cooled coil 9.

Part of the condensate is fed through a line 10 into the column 7 as reflux. The main part of the condensate is conveyed through a line 11 by means of a pump 12 into a cracking furnace 13 and converted therein into a synthesis gas mixture (N ₂ + 3H ₂ ) in a known manner. This gas mixture is introduced into a 1-sotope exchange tower 14 and exchanged there in countercurrent with liquid ammonia. This isotope exchange can only take place if the liquid ammonia contains a dissolved catalyst, eg KNH ₂ . The hydrogen in the gas mixture becomes depleted of deuterium and tritium, while the ammonia accumulates on deuterium and tritium.

15 ammonia is then formed from the gas mixture in a synthesis plant. The main part of the ammonia depleted in deuterium and tritium is conveyed by means of a pump 16 through a line 17 into the head part of the exchange tower 14.

The ammonia which is enriched in deuterium and tritium during the exchange with the synthesis gas mixture and in which a catalyst is dissolved is removed from the exchange tower 14 through a line 18, expanded in a throttle valve 19 and introduced into a concentrator 20.

The remaining part of the ammonia formed in the synthesis plant 15 is withdrawn through line 21 and a part thereof is expelled through line 22, in which a throttle valve 23 is arranged, into an evaporator 24. The ammonia vapor originating from this evaporator, which is depleted in deuterium and tritium, is introduced through a line 25 into an exchange tower 26 and is brought into isotope exchange therein in countercurrent with liquid ammonia enriched in deuterium and tritium. The ammonia vapor that accumulates here in deuterium and tritium is conveyed through a line 27 into a cooler 28 and condensed there by means of a cooling coil 29 through which water flows. The condensate is removed through a line 30 and conveyed into the concentrator 20 by means of a pump 31.

The concentrator 20 consists of a partial evaporator, not shown, which generates a catalyst-free ammonia vapor stream from the liquids fed through the lines 30 and 18 and a condenser, not shown, which liquefies this vapor stream.

The liquefied ammonia enriched in deuterium and tritium is removed from the concentrator 20 through a line 32.

The partial evaporator installed in the concentrator 20, on the other hand, still produces an ammonia liquid enriched in catalyst and deuterium and tritium, which is removed from the concentrator 20 through a line 33, expanded by a throttle valve 34 and into the top part of the isotope exchange tower 26.

The ammonia depleted in deuterium and tritium, which contains dissolved catalyst, is removed from the exchange tower 26 at the bottom through a line 35 and conveyed into the exchange tower 14 by means of a pump 36.

The ammonia stream withdrawn through line 21 of the ammonia synthesis 15, reduced by the partial amount withdrawn through line 22, is led away through line 37 and expanded through a throttle valve 38 into an evaporator 39. The ammonia vapor depleted in deuterium and tritium is then used to initiate the first ver driving step into the isotope exchange tower 3 fed through a line 40.

The free ammonium vapor enriched in deuterium and tritium from the concentrator 20 through line 32 is expanded through a throttle valve 41 into an evaporator 42. From here the ammonia vapor is fed through a line 43 into an isotope exchange tower 44. At the head of the exchange tower 44, a second water stream, which can originate from the same source as the first water stream, is fed in through a line 45 by means of a pump 46 and is counteracted in an isotope exchange with ammonia vapor analogously to the exchange tower 3. In contrast to the first process step, since the ammonia vapor is depleted in deuterium and tritium, the water is enriched in deuterium and tritium.

For the separation of the water vapor depleted of deuterium and tritium ammonia vapor is introduced into a rectification column 47 and the freed from water vapor, ammonia vapor in a condenser 48, in which a flow-through coolant line 49 is _- is arranged, liquefied. A portion of the condensate is returned through line 50 to the rectification column 47 as reflux, while the much larger amount of the condensate is withdrawn through line 51 and combined with the condensate withdrawn from condenser 9 through line 11.

A column 52 is arranged below the exchange tower 44 to separate the ammonia dissolved in the water, the steam required for the separation being generated with the aid of a heat source in the bottom of the column 52. This heat source can be designed, for example, as a steam-heated coil 53.

The water freed from ammonia and enriched in deuterium and tritium is removed through a line 54. This water can be used, for example, to feed a heavy water production plant or a tritium concentrating plant.

As already mentioned, FIG. 2 shows a flow diagram for a method in which water which is largely depleted in deuterium and tritium is also produced as the main product. As a further product, hydrogen enriched in deuterium and tritium is obtained and as a third product, nitrogen enriched in ¹⁵ N is produced.

The system elements in FIG. 2 which correspond to the system for carrying out a method according to FIG. 1 are designated with the same reference numerals to avoid repetitions, but which are provided with an apostrophe.

Their mode of operation corresponds to that of Fig. 1. For example, in an exchange column 3 ' River water or deuterium- and tritium-containing wastewater is introduced and removed as line at least largely freed from deuterium and tritium through line 6.

The entire plant is also independent of a synthesis plant for the industrial production of ammonia.

Instead of the plant according to FIG. 1, no catalyst-free ammonia enriched in deuterium and tritium is taken from a plant according to FIG. 2 and exchange medium used to produce deuterium and tritium-containing water is used.

Deviating from this measure, in the present case, an equivalent amount of liquid ammonia, which still contains dissolved catalyst, is withdrawn through a line 60 connected to the line 18 ′ and conveyed into an isotope exchange tower 62 by a pump 61. In the latter, the liquid ammonia is brought into isotope exchange in countercurrent with the synthesis gas mixture enriched in deuterium and tritium (N _Z + 3H _z ), the ammonia and the catalyst accumulating in deuterium and tritium, possibly up to pure ND ₃ or pure NT ₃ , while the synthesis gas mixture accumulates at ¹⁵ N.

The liquid depleted in deuterium and tritium and in ¹⁵ N in the isotope exchange tower 62 flows through a line 63 into a concentrator 64, which is similar to the concentrator 20 (FIG. 1) or 20 ′ (FIG. 2).

Two flows are generated in the concentrator 64, namely a liquid flow with a higher catalyst content than the liquid flow leaving the exchange tower 62. This liquid stream is enriched in deuterium and tritium and possibly contains ND ₃ and / or NT ₃ and is depleted of ¹⁵ N, possibly even completely ¹⁵ N free and is introduced through a line 65 into a concentrator 20 'after expansion in a throttle valve 66, whose mode of operation corresponds to that of the concentrator 20 in FIG. 1.

A second stream of liquid formed in the concentrator 64 consists of NH ₃ or ND ₃ and / or NT ₃ and is free of catalyst. In addition, it is impoverished at ¹⁵ N or even completely free of ¹⁵ N.

This second stream is conveyed by a pump 67 into a cracking furnace 68 and broken down therein into N _z + 3H _z or N ₂ + 3D ₂ and / or N ₂ + 3T _z .

The majority of these gases are returned through lines 69 and 70 to the isotope exchange tower 62, where they accumulate at ¹⁵ N and deplete in deuterium and tritium in the manner described above. The gas is then brought together through a line 71 with the synthesis gas originating from the cracking furnace 13 '.

The remaining amount of the fission products generated in the cracking furnace 68 is introduced through a line 72 into a hydrogen-nitrogen separation system 73 of a known type. For example, the process taking place in the separation plant can be carried out with the aid of low-temperature separation by liquefaction and distillation or by alternating, selective absorption or by selectively permeable membrane.

In the separating system 73, the introduced gas mixture is broken down into nitrogen and hydrogen, with line 74 withdrawing ¹⁵ N depleted or ¹⁵ N free hydrogen-free nitrogen. This nitrogen can continue to be used for industrial purposes.

A portion of the nitrogen-free hydrogen withdrawn through line 75, enriched in deuterium and tritium, can be withdrawn as product through line 76. There is then the possibility of enriching this product in a manner not shown with at least one further, subsequent stage of deuterium or tritium.

Another possibility is to burn the product with oxygen to water containing deuterium or tritium. The remaining partial flow is recirculated through a line 77 by means of a compressor 78 and mixed into the partial flow from line 69, the gas mixture being introduced through line 70 into the exchange tower 62.

The ammonia depleted in deuterium and tritium by line 37 ', but enriched in ^1s N, is expanded analogously to FIG. 1, evaporated and brought into isotope exchange with river water or river water containing deuterium or tritium, so that line 11' is flowed through by ammonia which is enriched at ¹⁵ N, the deuterium and tritium content corresponding to the deuterium and tritium concentration of the feed water.

The ¹⁵ N concentration of the synthesis gas mixture produced in the cracking furnace 13 ′ is relatively high when it emerges from the isotope exchange tower 14 ′, specifically because of the ¹⁵ N enrichment of the gas stream led out of the exchange tower 62, which is the same as that from the cracking furnace 13 'removed synthesis gas is admixed before entering the exchange tower 14'.

Part of the synthesis gas depleted in deuterium and tritium is branched off after the exchange tower 14 through a line 79 from the gas supplied to the ammonia synthesis system 15 'and introduced into a hydrogen-nitrogen separation system 80 of a known type.

The gas mixture is in the separation plant, which e.g. how the separation system 73 can be designed, broken down into two streams.

A first nitrogen stream enriched with ¹⁵ N, which is preferably hydrogen-free, is withdrawn through a line 81.

A second gas stream, which mainly consists of hydrogen (but is not necessarily nitrogen-free), is mixed through line 82 by means of a compressor 83 into the gas stream introduced into the exchange tower.

To compensate for the product stream withdrawn from the system through line 81 and the nitrogen withdrawn through line 74, nitrogen with a natural concentration of ^1s N is fed through line 84 into the system in the ^exemplary embodiment.

The line 84 could alternatively also not be connected to the entry into the exchange tower 14 ', but also to the gas entry of the exchange tower 62. The injection point of fresh nitrogen depends on the process optimization and especially on the desired ratio of the products ¹⁵ N, deuterium and tritium.

In summary, it can be stated that in the exemplary embodiment according to FIG. 2, river water or deuterium- and tritium-containing waste water and nitrogen gas are consumed, while the main products through line 6 'of deuterium and tritium are at least largely freed water, through line 81 nitrogen enriched with ¹⁵ N and Hydrogen enriched in deuterium and tritium can be withdrawn through line 76 and nitrogen, which is depleted in ¹⁵ N, is formed as a by-product, which is removed through line 74.

There is only an extremely low consumption of ammonia, essentially due to leaks in the system.

In conclusion, it should also be pointed out that it can be advantageous to connect several identical plants, be it according to FIG. 1 or be it according to FIG. 2 in series, in order, as required, to produce products which are strongly associated with the isotopes concerned are more depleted to win.

Such a series connection can also be replaced by a periodic mode of operation of a single installation, at least one product of a period being used to feed the subsequent period.

Are absolutely pure products, such as B. from deuterium and tritium completely freed water, pure isotope ¹⁵ N, etc. needed, the product withdrawals suitable final enrichment or depletion stages, as are common in practice, can be connected downstream.

Numerical example

Plant for the simultaneous depletion of deuterium and tritium and the extraction of water or heavy water enriched with deuterium.

Accepted services: 1. Tritium removal from wastewater:

3000 m ³ / year of wastewater with 200 Ci / m ³ tritium content, which arises from a nuclear fuel reprocessing plant of 1400 t / year, should be brought to the concentration of 0.03 Ci / m ³ permitted by law.

2. Deuterium-free water:

300 m ³ / h of water with less than 10 ppm D / D + H should be produced for agricultural purposes. The tritium concentration should be below 0.03 Ci / m ³ .

3. Heavy water:

The plant should produce as much deuterium-enriched water or D ₂ 0 as possible.

Solution of the task:

3000 m ³ / year waste water or 375 kg / h waste water with 200 Ci / m ³ T ₂ 0 and 150 ppm D / D + H are first mixed with 299 625 kg / h river water with 150 ppm D / D + H to get a Flow of 30,000 kg / h Form water with 0.25 Ci / m ³ T ₂ O and 150 ppm D / D + H.

This water is used as a feed (cf. FIG. 1, reference number “1” and FIG. 2, reference number “1 '”) for a plant for carrying out a method according to claim 1.

The following table shows numerical examples for three cases. Cases I and II correspond to all the features of claim 1, in that the ammonia mold throughput (see line "e") is greater than two thirds of the water mold throughput (see line "b"). Case III does not meet the aforementioned conditions in that the ammonia mold throughput is less than two thirds of the water throughput.

The numerical example shows that the feed water can be depleted to 10 ppm with only 20 separation stages if the mold throughputs are selected in accordance with the characterizing part of claim 1 (cases I and II).

The corresponding concentrations in line 40 (FIG. 1) and 42 '(FIG. 2) are 8.79 and 5.207 ppm, which is a depletion factor of 12.8 (case I) and 24.3 (case II) in the Tower 14 or 14 'corresponds. Case III, which does not meet the information regarding mold throughput according to claim 1, cannot achieve the proposed depletion to 10 ppm at all.

Even to reach 20 ppm, according to the numerical example, 50 separation stages were required instead of 20.

The concentration in line 40 (FIG. 1) or 42 ′ (FIG. 2) is 1.2252 ppm in case III, which corresponds to a depletion factor of 123.

Similar ratios apply to tritium, such as. can be seen from the table.

Claims (4)

1. A process for separating at least one heavy isotope from a compound containing hydrogen or from a mixture containing hydrogen, using an ammonia synthesis and a mixture of hydrogen and nitrogen, characterised in that in a first step of the process water containing deuterium and tritium is brought into isotopic exchange with ammonia vapour depleted of tritium and deuterium, the molar throughput of ammonia being chosen to be more than two-thirds the molar throughput of water, and during the isotopic exchange between the two streams the water is virtually entirely depleted of tritium and deuterium and is recovered as the, produkt, whereas the ammonia vapour becomes enriched with deuterium and tritium, but by the end of the first step of the process has a lesser concentration of deuterium and tritium than the water brought into isotopic exchange, and in that the ammonia vapour is then liquefied and in a second step of the process is broken down into a mixture comprising hydrogen and nitrogen, and in that in a third step of the process the gas mixture is depleted of deuterium and tritium by isotopic exchange with liquid ammonia that has been depleted of deuterium and tritium, the liquid ammonia having been obtained by synthesisation from the depleted gas mixture and containing a dissolved catalyst, and one portion of the synthesised ammonia being evaporated and returned as the exchange stream to the first stage of the process, whereas the other portion of the ammonia enriched in the third step of the process is processed further.

2. The process according to claim 1, characterised in that the portion of ammonia that has been enriched with deuterium and tritium is evaporated and brought into isotopic exchange with a second stream of water, the water being enriched with tritium and deuterium and recovered as a by-product, and in that the ammonia vapour that has been depleted of deuterium and tritium is liquefied and combined with the ammonia from the first step of the process before its breakdown into hydrogen and nitrogen.

3. The process according to claim 1, characterised in that at least one portion of the liquid ammonia that has been enriched with deuterium and tritium is brought into isotopic exchange with a second gas mixture of hydrogen and nitrogen that has been enriched with deuterium and tritium, and in doing so is further enriched with deuterium and tritium, the second gas mixture having resulted from breaking down the enriched ammonia, one portion of said gas mixture being produced as a deuterium and tritium enriched by-product, and in that the portion of the gas mixture that was depleted of deuterium and tritium during the isotopic exchange is combined with the gas mixture that was liquefied after the first step of the process and split into hydrogen and nitrogen.

4. The process according to claim 3, charac-. terised in that the deuterium and tritium enriched by-product is split into nitrogen and into hydrogen enriched with deuterium and tritium, and at least partially separated into pure nitrogen and pure. hydrogen, the pure nitrogen being removed, and one portion of the deuterium and tritium enriched hydrogen is used as the starting product for a further processing operation, while the larger portion is combined with the second gas mixture, and in that furthermore before the third step of the process nitrogen with a natural ^1sN concentration is combined with the first gas mixture and after the third step of the process one portion of the gas mixture that became enriched with ¹⁵N during the isotopic exchange is broken down into nitrogen enriched with ¹⁵N and into a stream consisting essentially of hydrogen, which stream is returned once more to the gas mixture stream before the third step of the process, while the nitrogen enriched with ¹⁵N is recovered as a (by-)product.

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